In some series hybrid vehicles, the vehicle has an internal combustion engine, a generator, a traction motor, a brake resistor, and a DC bus (“DC” means direct current) to which the generator, the traction motor, and the brake resistor are electrically coupled. The generator converts mechanical energy from the engine into electric energy on the bus and can operate in the opposite direction to convert electric energy from the bus into mechanical energy to assist the engine with other functions on board the vehicle (e.g., raising a bucket hydraulically). The traction motor converts electric energy from the bus into mechanical energy for use in driving one or more traction elements (e.g., ground-engaging wheels) (“motoring”) and can operate in the opposite direction to convert mechanical energy into electric energy on the bus (“electric braking”).
The DC bus voltage is subject to large transients introduced by transmission shifting and vehicle direction reversals (i.e., forward to reverse and reverse to forward). With respect to transmission upshifting, the motor is commanded to operate so as to decrease its speed and then increase its speed (e.g., in a V pattern) by first electric braking so as to supply electric energy to the bus and then motoring so as to remove electric energy from the bus. Transmission upshifting creates the most severe requirement on the DC bus. With respect to transmission downshifting, the motor is commanded to operate so as to increase its speed and then decrease its speed (e.g., in an inverted V pattern) by first motoring so as to remove electric energy from the bus and then electric braking so as to supply electric energy to the bus.
Regarding vehicle direction reversals, the vehicle may have a FNR control operable by the vehicle operator (“FNR” means forward, neutral, and reverse). If the FNR control is switched from forward to reverse or reverse to neutral, the motor is commanded to operate so as to decrease its speed to zero by electric braking so as to supply electric energy to the bus and then increase its speed by motoring so as to remove electric energy from the bus.
It is known for a generator controller to receive voltage readings of the DC bus voltage of the DC bus and to control the generator to try to maintain the DC bus voltage at a nominally constant voltage (“nominal DC bus voltage” or “nominal voltage”) by use of closed-loop voltage control, such as PI-based voltage control (“PI” means Proportional/Integral), assisted by hysteretic control of the brake resistor. Using such prior art PI-based voltage control scheme, the generator controller operates the generator in a generating mode to convert mechanical energy into electric energy so as to supply electric energy to the DC bus, or a motoring mode to convert electric energy from the DC bus into mechanical energy so as to remove electric energy from the DC bus to assist the engine. If the rotational speed of the engine reaches a speed threshold, due, for example, to the additional energy from operation of the generator in the motoring mode, the generator controller ceases or otherwise prevents operation of the generator in the motoring mode.
Meanwhile, the DC bus voltage of the DC bus is monitored. According to the prior art hysteretic control scheme, if the DC bus voltage exceeds a DC bus threshold (e.g., due to electric braking of the motor without sufficient motoring of the generator), the brake resistor is operated in a constant ON state to dissipate electric energy from the DC bus. If the DC bus voltage is lower than the DC bus threshold, the brake resistor is in a constant OFF state.